Therapy delivery for identified tachyarrhythmia episode types

ABSTRACT

Methods and systems for identifying tachyarrhythmia episode types and delivering therapy to mitigate the identified tachyarrhythmia episode types are described. Electrogram signals of cardiac activity are sensed and stored by an implantable cardiac device. Tachyarrhythmia episodes are detected and tachyarrhythmia episode types are identified based on characteristics of the electrogram signals. In preparation for performing ablation, a tachyarrhythmia episode is induced. The features of the induced tachyarrhythmia episode are compared to characteristics of the identified episode types. A similarity between the induced tachyarrhythmia episode and at least one of the episode types identified from the stored electrogram signals is indicated to facilitate performing the ablation.

RELATED APPLICATIONS

This application claims the benefit of Provisional Patent ApplicationSer. No. 60/861,594, filed Nov. 29, 2006, to which priority is claimedpursuant to 35 U.S.C. §119(e) and which is hereby incorporated herein byreference.

FIELD OF THE INVENTION

The present invention relates generally to cardiac devices and methods,and, more particularly, to identifying tachyarrhythmia episode types anddelivering therapy to mitigate the identified tachyarrhythmia episodetypes.

BACKGROUND OF THE INVENTION

Proper cardiac function relies on the synchronized contractions of theheart at regular intervals. When the heart is functioning normally,synchronized cardiac contractions are initiated at the sinoatrial nodeand the heart is said to be operating in normal sinus rhythm. However,if contractions of the heart become irregular or uncoordinated, or ifthe contraction rate is too fast or too slow, the heart rhythm isdescribed as arrhythmic. Cardiac arrhythmia may be caused, for example,by disease processes or from aberrant electrical conduction patternsoccurring in the heart tissue. Cardiac arrhythmia impairs cardiacpumping efficiency and some types of cardiac arrhythmia can be lifethreatening.

A cardiac arrhythmia that originates in an atrial region of the heart isdenoted a supraventricular tachyarrhythmia (SVT). Atrial fibrillationand atrial flutter are examples of SVT. Both conditions arecharacterized by rapid, uncoordinated contractions of the atriaresulting in hemodynamically inefficient pumping action.

Another example of SVT is sinus tachycardia, which is an increased heartrate due to exercise or a quick emotional response. In contrast toatrial fibrillation and atrial flutter, sinus tachycardia ischaracterized by rapid, coordinated contractions of the atria resultingin hemodynamically efficient pumping action, compensating for theincreased strain placed upon the body during exercise or quick emotionalresponses. Whereas atrial fibrillation and atrial flutter are “abnormal”(yet not lethal), sinus tachycardia is “normal” (and also not lethal).

Cardiac arrhythmias originating in a ventricular region of the heart aredenoted ventricular tachyarrhythmias. Ventricular tachycardia (VT) ischaracterized by rapid ventricular contractions and can degenerate intoventricular fibrillation (VF). Ventricular fibrillation producesextremely rapid, non-coordinated contractions of the ventricles.Ventricular fibrillation is fatal unless the heart is returned to sinusrhythm within minutes or even seconds.

Implantable cardiac devices, including pacemakers and implantablecardioverter/defibrillators (ICDs), and have been used to delivereffective treatment to patients with serious cardiac arrhythmias.Implantable cardiac devices may treat cardiac arrhythmias with a varietyof tiered therapies. These tiered therapies range from delivering lowenergy pacing pulses timed to assist the heart in maintaining pumpingefficiency to providing high-energy shocks to treat and/or terminatefibrillation. To effectively deliver these treatments, the cardiacdevice must first identify the type of arrhythmia that is occurring,after which appropriate therapy may be delivered to the heart.

Some tachyarrhythmias are caused by abnormal cardiac tissue that createsshort circuits in the electrical conduction pathways of the heart.Ablation is a therapeutic procedure that destroys the abnormal tissue toprevent or reduce recurrence of these types of tachyarrhythmias.Ablation may be used in conjunction with an ICD to reduce the number ofshocks delivered to the heart to terminate VT or VF.

Methods and systems that facilitate identification of the causes oftachyarrhythmia episodes aid in determining appropriate therapies totreat the disorders causing the episodes. The present invention fulfillsthese and other needs.

SUMMARY OF THE INVENTION

The present invention is directed to methods and systems for identifyingtachyarrhythmia episode types and delivering therapy to mitigate theidentified tachyarrhythmia episode types. One embodiment is directed toa method of operating a cardiac system. Electrogram signals of cardiacactivity are sensed and stored by an implantable cardiac device.Tachyarrhythmia episodes are detected using the electrogram signals.Tachyarrhythmia episode types are identified based on characteristics ofthe stored electrogram signals. In preparation for performing ablation,a tachyarrhythmia episode is induced. The features of the inducedtachyarrhythmia episode are compared to characteristics of theidentified episode types. A similarity is indicated between the inducedtachyarrhythmia episode and at least one of the episode types identifiedfrom the stored electrogram signals to facilitate performing theablation.

The characteristics of the tachyarrhythmia episodes of each episode typeare associated with a conduction pattern of the episode type. Thecharacteristics may involve one or more morphological characteristicsand/or one or more interval characteristics of the tachyarrhythmiaepisodes.

In a further approach, the number of tachyarrhythmia episodes associatedwith each episode type are counted and displayed. The episode types maybe ranked, where the rank of an episode type corresponds to atachyarrhythmia burden of the episode type. The criteria used for theranking is user programmable.

Ablation may be performed to prevent or mitigate future occurrences ofone or more of the identified episode types. Episode types identifiedfrom tachyarrhythmia episodes stored before ablation may be compared toepisode types identified from tachyarrhythmia episodes stored afterablation to confirm success of the ablation.

The number of distinct episode types may be determined algorithmicallyby an implantable or patient-external device or may be determined by ahuman analyst. In one implementation, after arranging the episodes in anorder, the human analyst may identify one or more boundary episodesbetween the episode types. The tachyarrhythmia episodes may be groupedinto episode types based on the boundary episodes.

In one implementation, a far field electrogram signal and a near fieldelectrogram signal are sensed and stored. The characteristicsidentifying tachyarrhythmia episode types comprise morphologicalcharacteristics determined using the far field signal and the near fieldsignal. In another implementation, a plurality of far field electrogramsignals and a plurality of near field electrogram signals are sensed andstored. The characteristics identifying tachyarrhythmia episode typesare determined using at least one of the far field signals and at leastone of the near field signals.

In another approach, exit sites of the identified episode types aredetermined using the electrogram signals.

Another embodiment of the invention is directed to a cardiac system. Thesystem includes an implantable sensing system and memory configured tosense and store electrogram signals. A tachyarrhythmia detector detectstachyarrhythmia episodes using the electrogram signals and determinescharacteristics of the tachyarrhythmia episodes from the electrogramsignals. A data processor identifies episode types based on thecharacteristics of the tachyarrhythmia episodes. The data processor alsodetermines a similarity between a tachyarrhythmia episode induced inpreparation for performing ablation and at least one identified episodetype. A display presents information related to the similarity betweenthe induced tachyarrhythmia episode and the at least one identifiedepisode type to facilitate performing the ablation.

The characteristics used to identify the episode types may includemorphological characteristics or interval characteristics of theelectrogram signals. The data processor may be configured to determine anumber of distinct episode types and/or may discriminate a first type ofventricular tachyarrhythmia episode from a second type of ventriculartachyarrhythmia episode.

According to various implementations, the data processor may count thetachyarrhythmia episodes of the various episode types and/or may rankthe episode types according to the tachyarrhythmia burden of the episodetypes.

In some implementations, the sensing system includes a first pair ofcardiac electrodes configured to sense a far field electrogram signaland a second pair of cardiac electrodes configured to sense a near fieldelectrogram signal. The tachyarrhythmia detector determines thecharacteristics of the tachyarrhythmia episodes using the far fieldelectrogram signal and the near field electrogram signal.

In some implementations, the sensing system includes a plurality of farfield cardiac electrode pairs, each far field electrode pair configuredto sense a far field electrogram signal, and a plurality of near fieldcardiac electrode pairs, each near field electrode pair configured tosense a near field electrogram signal. The tachyarrhythmia detectordetermines the characteristics of the tachyarrhythmia episodes using atleast one of the plurality of far field electrogram signals and at leastone of the plurality of near field electrogram signals.

The data processor may be further configured to determine an exit siteof one or more of the episode types based on the sensed and storedelectrogram signals and/or may identify the exit site through comparisonof the characteristics associated with a particular episode type toelectrogram signals of paced beats.

The above summary of the present invention is not intended to describeeach embodiment or every implementation of the present invention.Advantages and attainments, together with a more complete understandingof the invention, will become apparent and appreciated by referring tothe following detailed description and claims taken in conjunction withthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a diagram illustrating a process of determining oridentifying tachyarrhythmia episode types and comparing identifiedepisode types to an induced tachyarrhythmia episode in preparation forablation in accordance with embodiments of the invention;

FIG. 1B is a diagram illustrating a process for identifyingtachyarrhythmia episode types and determining a tachyarrhythmia burdenof episode types in accordance with embodiments of the invention;

FIG. 2 shows an implantable cardiac device configured to acquire datarelated to cardiac episodes in accordance with embodiments of theinvention;

FIG. 3 depicts a block diagram of a system suitable for acquiringcardiac episode data, for identifying cardiac episodes, and fordetermining the tachyarrhythmia burden associated with the cardiacepisodes in accordance with embodiments of the present invention;

FIG. 4 is a diagram illustrating a patient-external device that providesa user interface allowing a human analyst to interact with the cardiacepisode data in s accordance with embodiments of the invention;

FIGS. 5A and 5B are diagrams illustrating near field and far fieldcardiac electrogram signals that may be used to identify cardiactachyarrhythmia episodes in accordance with embodiments of theinvention;

FIG. 6 is a diagram illustrating a process for labeling episode typesand counting tachyarrhythmia episodes associated with the episode typesbased on near field and far field electrogram signals in accordance withembodiments of the invention; and

FIG. 7 is diagram illustrating a process for determining exit sites forvarious tachyarrhythmia episode types in accordance with embodiments ofthe invention.

While the invention is amenable to various modifications and alternativeforms, specifics thereof have been shown by way of example in thedrawings and will be described in detail below. It is to be understood,however, that the intention is not to limit the invention to theparticular embodiments described. On the contrary, the invention isintended to cover all modifications, equivalents, and alternativesfalling within the scope of the invention as defined by the appendedclaims.

DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS

In the following description of the illustrated embodiments, referencesare made to the accompanying drawings forming a part hereof, and inwhich are shown by way of illustration, various embodiments by which theinvention may be practiced. It is to be understood that otherembodiments may be utilized, and structural and functional changes maybe made without departing from the scope of the present invention.

Tachyarrhythmias such as ventricular tachycardia (VT) or prematureventricular contractions (PVCs) can trigger ventricular fibrillation(VF) and sudden cardiac death. Implantable cardiac devices, e.g.,implantable cardioverter/defibrillators (ICDs) treat VT and/or VF bydelivering therapy in the form of anti-tachycardia pacing (ATP) and/orhigh energy defibrillation shocks. An interventional cardiacelectrophysiologist can use catheter ablation techniques to ablateabnormal tissue causing PVCs or VT which may result in a reduced needfor ICD therapy.

Some tachyarrhythmia episodes exhibit consistent morphology or intervalpatterns that may be discerned from the electrograms (EGMs) sensedduring the episodes. Tachyarrhythmia episodes exhibiting consistent EGMpatterns may be identified as belonging to particular episode typesbased on characteristics evident in the cardiac electrical signals ofthe tachyarrhythmia episodes. For example, the cardiac signals oftachyarrhythmia episodes of a first episode type exhibit representativemorphological or interval characteristics that are distinct from themorphological or interval characteristics of tachyarrhythmia episodes ofa second episode type. In one implementation, analysis of theelectrogram signals by an implantable or patient-external device may beused to discriminate between distinct ventricular tachyarrhythmiaepisode types that exhibit consistent morphological or intervalpatterns. In addition, various types of ventricular tachyarrhythmias maybe discriminated from episode types that are non-ventricular in origin,denoted herein as supraventricular tachyarrhythmias (SVTs).

Therapy to treat abnormal cardiac rhythms, including ablation,anti-tachyarrhythmia pacing (ATP), and/or defibrillation therapy, may beenhanced by identifying tachyarrhythmia episode types. For example,analysis of EGM signals collected by an ICD may be used to identifytachyarrhythmia episode types that historically have been experienced bythe patient. Characteristics of an induced tachyarrhythmia episode maybe compared to the identified episode types prior to ablation.Similarities between the induced tachyarrhythmia episode and theidentified episode types may be used in determining or confirmingablation sites.

Episode types that are more problematic for the patient may includeepisode types that occur more frequently, are sustained, require therapydelivery for termination, are highly unstable, and/or that typicallyaccelerate to VF. Embodiments of the invention are directed to methodsand systems that determine the “tachyarrhythmia burden” as a way ofranking the tachyarrhythmia episodes types according to predeterminedcriteria. For example, the episode types may be ranked based on thenumber or frequency of occurrence of episode types. In someimplementations, the episode types may be ranked based on factors suchas the instability of the episode types, the amount of morphologicaldisorganization of the episode type, the tendency of the episode type toaccelerate to VF, the responsiveness of the episode type treatment suchas ATP, and/or other factors.

One embodiment of the invention is illustrated in the diagram of FIG.1A. An ICD includes electrodes disposed in appropriate locations in, on,or about the heart for sensing cardiac electrical activity. Via theelectrodes and sensing circuitry disposed within the ICD housing, thedevice senses 110 electrogram (EGM) signals of cardiac electricalactivity. The EGM signals may be stored in the ICD for a period of timeand/or may be transmitted to a patient-external device via a telemetriclink.

ICDs typically include cardiac electrodes that are in electrical contactwith the myocardium and sense cardiac electrical signals that provideinformation about the depolarization status of the heart. Near fieldsensing yields signals that are most strongly representative of theactivation signals that are present close to the site of an electrodepair.

Far field signals provide a more global view of the depolarizationstatus of the heart than near field signals. For example, a sensed farfield cardiac activation signal is effectively a superposition of anumber of near field depolarization signals occurring within the heartthat are associated with a cardiac contraction.

ICDs that include pacing capability typically include tip electrodesthat are configured to make direct contact with the myocardium. Tipelectrodes may be used for near field sensing of cardiac electricalsignals. Far field sensing may be accomplished via various electrodepairs of an ICD, such as transvenous, endocardial, and/or epicardialelectrodes (i.e., intrathoracic electrodes), and/or subcutaneous,non-intrathoracic electrodes, including can, header, and indifferentelectrodes, and subcutaneous array or lead electrodes (i.e.,non-intrathoracic electrodes). For example, electrode pairs suitable forfar field sensing in an ICD may include RV-ring to RV-coil, RV-ring toSVC-coil, RV-ring to RA-ring, or may include sensing between two canelectrodes, between a can electrode and an indifferent electrode, orbetween a can or indifferent electrode and a ring or coil electrode.ICDs capable of biventricular pacing provide additional near field andfar field sensing capabilities. An electrode pair used for sensingcardiac electrical activity is referred to herein as a sensing vector. Asensing vector includes at least a pair of sensing electrodes, whereeach electrode of the pair may comprise multiple electrodes and/ormultiple electrode elements used for sensing.

Returning now to FIG. 1A, near field and/or far field cardiac electricalsignals are sensed by the ICD. Tachyarrhythmia episodes are detected 120from the EGM signals. The EGM signals of the tachyarrhythmia episodesmay be stored in the memory of the ICD, optionally along with dataacquired from additional physiological or non-physiological sensors. Theadditional data may include information such as time and day of theepisode, episode onset, episode duration, morphological organizationassociated with the episode, therapy delivered, therapy success, and/orother data.

Tachyarrhythmia episode types are identified 130 through analysis of theEGM signal features of the tachyarrhythmia episodes. Each episode typeis associated with a certain morphology or interval pattern of the EGMsignal, which in turn is indicative of a conduction pattern associatedwith the episode type. Each episode type detected by the device is givena label, e.g., VT001 or PVC001.

During pre-ablation mapping, tachyarrhythmia episodes are induced 140.Features of the induced tachyarrhythmia episodes are compared to thecharacteristic features of the identified episode types. The similaritybetween the induced episode and at least one identified episode type isdetermined 150. The similarity may be displayed or otherwise indicatedto facilitate the ablation.

Another process in accordance with embodiments of the invention isillustrated in the diagram of FIG. 1B. Electrogram signals are sensed115 and stored in an ICD. Tachyarrhythmia episodes are detected 125 fromthe stored electrogram signals. Tachyarrhythmia episode types areidentified 135 through analysis of the EGM signal features of thetachyarrhythmia episodes. The tachyarrhythmia episodes associated witheach episode type are counted 145. The numbers of tachyarrhythmiaepisodes associated with the episode types are displayed 155.

One approach to detecting tachyarrhythmia episodes and classifyingepisode types is based on sensed event intervals, e.g., the P-P, P-R,R-P, or R-R timing intervals, and/or the atrial heart rate compared toventricular heart rate along with the stability of the intervals and/orthe onset characteristics of the intervals. Event intervals and ratesmay be identified as they occur and are compared to preset criteria,which must be met in order to identify a particular tachyarrhythmiatype. U.S. Pat. No. 5,342,402, which is incorporated herein byreference, describes an interval based tachyarrhythmia detection andclassification system and method.

In other approaches, the morphology of the EGM signal of arepresentative beat of the tachyarrhythmia episode may be analyzed todetect tachyarrhythmia and to classify tachyarrhythmia episode types.For example, morphological features extracted from the EGM signals maybe compared to a template including corresponding featuresrepresentative of a particular type of tachyarrhythmia. The featuresextracted from the EGM signal may include the time coordinates of localmaxima and minima points of the cardiac signal, for example. If thesignal features are sufficiently similar to the template features, thetachyarrhythmia episode may be identified as the episode typerepresented by the template. Additional details related to extraction offeatures points from cardiac signals and the use of such feature pointsto discriminate between various cardiac rhythms is described in commonlyowned U.S. Pat. Nos. 6,266,554 and 6,449,503 which are incorporatedherein by reference.

In yet another embodiment, the electrogram signal characteristics usedto identify tachyarrhythmia episode types are coefficients of Fourier orwavelet decomposition of a composite cardiac beat signal for theepisode. In one implementation, the composite beat signal of an episodeis transformed into a number of signal wavelet coefficients using awavelet transform, such as a Haar wavelet transform. The higheramplitude signal wavelet coefficients are identified and used asdiscriminating features of the episode. Additional details regarding theuse of wavelet transformation to extract wavelet coefficients of cardiacelectrogram signals are described in U.S. Pat. No. 6,393,316 which isincorporated herein by reference.

In yet a further embodiment, the features of the electrograms used toclassify tachyarrhythmia episode types include areas between sections ofan EGM signal (which may be a composite signal) and a baseline. Forexample, a group of consecutive peaks having the largest cumulative peakvalues are determined from the EGM signal. Features of the peaks such asareas of each peak, are determined and used to discriminate betweendifferent types of tachyarrhythmia. Additional features used fordiscriminating episode types may comprise the polarity and/or positionof the peak. Further discussion regarding the extraction of peakinformation for use as discriminating features of a cardiac EGM signalare discussed in U.S. Pat. No. 5,779,645 which is incorporated herein byreference.

In some configurations, the episode types are ranked and the rankingsare displayed. For example, the rankings may be displayed along with thenumber of episodes counted for each episode type. Counting or rankingthe episode types provides a physician with an assessment of thetachyarrhythmia burden associated with each episode type. Furthermore,identifying and/or ranking tachyarrhythmia episode types may be used toprovide the electrophysiologist with a pre-ablation map of PVC or VTpattern “signatures” of frequently occurring or otherwise problematictachyarrhythmia episode types.

One implementation in accordance with embodiments of the inventioninvolves ranking episode types based on the number of times atachyarrhythmia episode of a particular episode type occurs. Forexample, the episode types may be ranked according to the number oftachyarrhythmia episodes counted for each episode type. The episode typewith the highest count is ranked as having the highest tachyarrhythmiaburden. The episode type having the lowest count is ranked lowest.

In other implementations, information in addition to the EGM signals maybe stored and linked to the episode types. The additional informationmay be used in ranking the episode types. In these implementations, theepisode types may be ranked according to onset characteristics,stability characteristics, duration, success or failure of therapiesused for treatment, or other characteristics. The characteristics usedfor ranking may be programmable.

By identifying and labeling specific episode types, theelectrophysiologist will also be able to identify supraventriculartachyarrhythmias (SVTs) that may be conducted aberrantly. If notidentified, supraventricular tachyarrhythmias, which have their owncharacteristics, may cause an inappropriate shock. Therefore,identification of an SVT based on characteristics of the EGM wouldresult in therapy inhibition the next time this SVT occurs. Using thisimplementation, specific therapies, or withholding of nominallyprogrammed therapies, can be programmed according to the tachyarrhythmiaidentified. For example, an SVT that is conducted aberrantly (i.e.unlike the stored sinus rhythm template) can be identified by the devicewhich can be programmed to inhibit therapy for the episode type.Similarly, a specific VT can be programmed to be treated withanti-tachycardia pacing even if the rate identifies it as atachyarrhythmia in the VF zone. Similarly, tailored therapy withspecific ATP sequences using a specific left ventricular, rightventricular, or biventricular stimulation or immediate shock delivery,regardless or rate can be assigned to any type of tachyarrhythmia typeobserved.

Information provided by ranking the episode types may be used by anelectrophysiologist in performing ablation to prevent future occurrencesof certain episode types. For example, it may be desirable to performablation to eliminate one or more episode types associated with thehighest tachyarrhythmia burden. In another implementation, ranking thetachyarrhythmia episode types may be used by a physician to adjust theprogrammable settings of an ICD to enhance electrical stimulationtherapies delivered by the device, or may be used for other therapeuticor diagnostic purposes. Information about the tachyarrhythmia episodes,including the rankings of the tachyarrhythmia episodes before and afterablation is performed and/or before and after the ICD settings areadjusted, may be displayed and/or compared.

Referring now to FIG. 2 of the drawings, there is shown an ICDconfigured to implement methods in accordance with various embodimentsof the present invention. The ICD 200 in FIG. 2 includes pacemaker anddefibrillator circuitry enclosed within a housing and coupled to a leadsystem 202. The housing and/or header of the device 200 may incorporateone or more can or indifferent electrodes 208, 209 used to provideelectrical stimulation energy to the heart and/or to sense cardiacelectrical activity. The ICD 200 may utilize all or a portion of thedevice housing as a can electrode 208 and/or may have multiple canelectrodes disposed on the housing. The ICD 200 may include one or moreindifferent electrodes 209 positioned, for example, on the header or thehousing of the ICD 200.

The lead system 202 is used to sense cardiac electrical signals producedby the heart and to provide electrical energy to the heart under certainpredetermined conditions to treat cardiac arrhythmias. The lead system202 may include one or more electrodes used for pacing, sensing, and/ordefibrillation. In the embodiment shown in FIG. 2, the lead system 202includes an intracardiac right ventricular (RV) lead system 204, anintracardiac right atrial (RA) lead system 205, and an intracardiac leftventricular (LV) lead system 206. An extracardiac left atrial (LA) leadsystem 207 may optionally be employed.

The ICD 200 and lead system 202 illustrated in FIG. 2 may be configuredfor biventricular or biatrial sensing and/or pacing. The lead system 202of FIG. 2 illustrates one embodiment that may be used in connection withthe processes described herein. Other leads and/or electrodes mayadditionally or alternatively be used. For example, the lead system 202may include multiple electrodes in one chamber configured forintrachamber pacing and sensing. In this configuration, the ICD 200 maypace and/or sense at multiple sites in one cardiac chamber via multipleelectrodes within the chamber. This type of multisite pacing and sensingmay be employed in one or more of the right atrium, left atrium, rightventricle or left ventricle. Multisite pacing in a chamber may be usedfor example, to increase the synchrony of cardiac contractions of thepaced chamber.

As illustrated in FIG. 2, the lead system 202 may include one or moreextracardiac leads 207 having electrodes 215, 218, e.g., epicardialelectrodes, patch electrodes or other types of extracardiac electrodespositioned at locations outside the heart for sensing and pacing one ormore heart chambers. In various configurations, the epicardialelectrodes may be placed on or about the outside of the heart and/or maybe embedded in the myocardium from the locations outside the heart.

The right ventricular lead system 204 illustrated in FIG. 2 includes anSVC-coil 216, an RV-coil 214, an RV-ring electrode 211, and an RV-tipelectrode 212. The right ventricular lead system 204 extends through theright atrium and into the right ventricle. In particular, the RV-tipelectrode 212, RV-ring electrode 211, and RV-coil electrode 214 arepositioned at appropriate locations within the right ventricle forsensing right ventricular cardiac signals and delivering electricalstimulation pulses to the heart. The SVC-coil 216 is positioned at anappropriate location within the right atrium chamber of the heart or amajor vein leading to the right atrial chamber.

In one configuration, the RV-tip electrode 212 referenced to the canelectrode 208 may be used to implement unipolar pacing and/or near fieldsensing in the right ventricle. Bipolar pacing and/or sensing in theright ventricle may be implemented using the RV-tip 212 and RV-ring 211electrodes. In yet another configuration, the RV-ring 211 electrode mayoptionally be omitted, and bipolar pacing and/or sensing may beaccomplished using the RV-tip electrode 212 and the RV-coil 214, forexample. The right ventricular lead system 204 may be configured as anintegrated bipolar pace/shock lead. The RV-coil 214 and the SVC-coil 216are defibrillation electrodes.

The left ventricular lead 206 includes an LV distal electrode 213 and anLV proximal electrode 217 located at appropriate locations in or aboutthe left ventricle for pacing and/or sensing the left ventricle. Theleft ventricular lead 206 may be guided into the right atrium of theheart via the superior vena cava. From the right atrium, the leftventricular lead 206 may be deployed into the coronary sinus ostium, theopening of the coronary sinus 250. The lead 206 may be guided throughthe coronary sinus 250 to a coronary vein of the left ventricle. Thisvein is used as an access pathway for leads to reach the surfaces of theleft ventricle which are not directly accessible from the right side ofthe heart. Lead placement for the left ventricular lead 206 may beachieved via subclavian vein access and a preformed guiding catheter forinsertion of the LV electrodes 213, 217 adjacent to the left ventricle.

Unipolar pacing and/or near field sensing in the left ventricle may beimplemented, for example, using the LV distal electrode 213 referencedto the can electrode 208. The LV distal electrode 213 and the LVproximal electrode 217 may be used together as bipolar sense and/or paceelectrodes for the left ventricle. The lead system 202 in conjunctionwith the device 200 may provide bradycardia pacing therapy to maintain ahemodynamically sufficient heart rate. The left ventricular lead 206 andthe right ventricular lead 204 and/or the right atrial lead and the leftatrial lead may be used to provide cardiac resynchronization therapysuch that the ventricles and/or atria of the heart are pacedsubstantially simultaneously or in phased sequence separated by aninterventricular or interatrial pacing delay, to provide enhancedcardiac pumping efficiency for patients suffering from congestive heartfailure.

The right atrial lead 205 includes a RA-tip electrode 256 and an RA-ringelectrode 254 positioned at appropriate locations in the right atriumfor sensing and pacing the right atrium. In one configuration, theRA-tip 256 referenced to the can electrode 208, for example, may be usedto provide unipolar pacing and/or sensing in the right atrium. Inanother configuration, the RA-tip electrode 256 and the RA-ringelectrode 254 may be used for bipolar pacing and/or sensing.

The ICD 200 can be programmed to acquire the near field and/or far fieldcardiac signals sensed via electrodes 211-218, 254, 256, 208 and 209during cardiac tachyarrhythmia episodes. Near field sensing vectorsinclude, for example, the signals recorded from a tip electrode. Farfield sensing vectors may include the bipolar signals recorded from thering of a pacing lead to the device canister and/or defibrillationcoils. The number of far field signals recorded is a function of thenumber of electrodes included in the system.

The device circuitry includes a memory 245 for storing cardiac signalsand/or other data, including data related to tachyarrhythmia episodes.The ICD 200 also includes communication circuitry (not shown) tofacilitate wireless communication with a patient-external device, suchas a device programmer and/or remote server. The cardiac signals and/orother data may be acquired and stored in the ICD memory for a period oftime. The stored signals and data may be downloaded periodically or oncommand to a remote system via the communications circuitry.

Although the ICD 200 is described as an implantable therapy device, thedevice that acquires the episode data need not be implantable and neednot have therapy capability. For example, the principles of theinvention are also applicable using a cardiac monitor that ispatient-external and only acquires and optionally stores cardiac signalswithout delivering therapy.

Referring now to FIG. 3, there is shown a block diagram of a systemsuitable for identifying tachyarrhythmia episode types, determiningsimilarities between induced tachyarrhythmia episodes and previouslyidentified episode types. Components of the system 300 of FIG. 3 mayoperate to count tachyarrhythmia episodes and/or rank episode type inaccordance with various embodiments of the invention. FIG. 3 shows asystem 300 divided into functional blocks. It is understood by thoseskilled in the art that there exist many possible configurations inwhich these functional blocks can be arranged. The example depicted inFIG. 3 is one possible functional arrangement. Other arrangements arealso possible. For example, more, fewer or different functional blocksmay be used. Although the system depicted in FIG. 3 contemplates the useof programmable microprocessor-based logic circuits, other circuitimplementations may be utilized. Further, the system 300 illustrates animplementation that includes implantable components 370 andpatient-external components 360. It will be understood that in otherimplementations more, fewer or different components may be implementedas patient-external components and/or more, fewer or differentcomponents may be implemented as implantable components.

The system 300 includes implantable electrodes 305 coupled to sensingcircuitry 310 through a switch matrix 325. The electrodes 305 may bedisposed implantably at multiple locations within, on, or about theheart, may be disposed subcutaneously, e.g., on the surface of animplantable device housing, and/or may be arranged patient-externally onthe surface of the patient's skin, as previously described. Variouscombinations of the electrodes coupled via the switch matrix 325 to thesensing circuitry 310 may be used to sense near field and/or far fieldcardiac electrical signals.

Control processor 340 may include circuitry for triggering dataacquisition before, during and/or after the occurrence of a cardiactachyarrhythmia episode. Collection (i.e., acquisition and storage) ofcardiac signal data and optionally other data only during time windowsaround episode occurrences may be useful in devices where memory islimited. Alternatively, if the system includes sufficient memory,cardiac signals may be continuously collected for later analysis todetect episode occurrences. Alternatively, the device may continuouslyor intermittently transmit the data in real time as it is acquired to aremote device.

The system may optionally have therapy capability. In therapy-capableimplementations, the control processor 340 controls therapy circuitrysuch as pulse generator circuitry 330. The pulse generator circuitry 330has the ability to generate pacing pulses for treating bradycardiaand/or the ability to generate anti-tachyarrhythmia pacing pulses and/orhigh energy defibrillation or cardioversion shocks used for terminatingdangerous tachyarrhythmias such as VF.

In various embodiments, the system acquires and stores cardiac signalssensed via the electrodes 305 continuously or during a time windowbefore, during, and/or after cardiac tachyarrhythmia episodes or duringother times, such as following LV or RV pacing. In one example, cardiacsignals from each cardiac episode experienced by the patient areacquired along with one or more of cardiac marker channel signals. Thesignals associated with the episode are stored in the memory 345 and maybe time and date stamped.

In one implementation, the cardiac electrodes 305, sense circuitry 310,memory 345, and control processor 340 are components of an implantabledevice, e.g., ICD 307. In this implementation, the episode datacollected during one or more cardiac episodes may be transferred to apatient-external device 360 to facilitate interaction with a humananalyst. For example, the ICD 370 may include a transmitter/receiver 350configured to transmit the stored episode data to the patient-externaldevice 370 automatically, periodically, or on command to atransmitter/receiver 362 of the patient-external device 360. Inaddition, data and/or program commands useful for controlling theoperation of various components of the ICD 370 may be transmitted viathe patient external device 360 and stored in the memory 345 of the ICD370.

The patient external device 360, e.g., device programmer or remoteserver, optionally includes a tachyarrhythmia detector 364 configured todetect tachyarrhythmia episodes from the cardiac electrical signalsdownloaded from the ICD 370. The tachyarrhythmia detector 364 is furtherconfigured to determine characteristics of the tachyarrhythmia episodesfrom the electrogram signals. A data processor 366, optionally includedin the patient external device 360, identifies episode types based onthe morphological or interval characteristics of the tachyarrhythmiaepisodes. The number of different episode types into which the sensedtachyarrhythmia episodes are grouped may be determined algorithmicallyby the data processor 366, or may be input by a user. Although FIG. 3illustrates the tachyarrhythmia detector 364 and data processor 366 ascomponents of the patient-external device 360, alternatively, thesefunctions could be implemented in the implanted device 370 or could beimplemented partially in the implanted device 370 and partially in thepatient external device 360.

Arrhythmia induction circuitry 390 is used to induce one moretachyarrhythmia episodes, such as during a pre-ablation procedure.Electrograms of the induced tachyarrhythmias are sensed via the cardiacelectrodes 305 and sense circuitry 310. Features of the inducedtachyarrhythmia episodes are compared to those of the identified episodetypes. If the features of an induced tachyarrhythmia episode are similarto the features associated with an identified episode type, then thisinformation may be used by the electrophysiologist to inform theablation process. For example, the electrophysiologist may beparticularly interested in identifying cardiac sites that areresponsible for episode types producing the greatest tachyarrhythmiaburden. Ablation of these sites may be used to reduce or eliminates amost frequently occurring episode type or an episode type that isparticularly problematic for the patient.

In some embodiments, the data processor 366 includes counter circuitryconfigured to count the number of tachyarrhythmia episodes for eachepisode type. The data processor 366 may optionally rank the episodetypes based on the number of tachyarrhythmia episodes counted for eachepisode type or based on other information. The count or rank oftachyarrhythmia episodes for each episode type and/or informationrelated to similarity between induced tachyarrhythmia episode andidentified episode types is transferred to a display device 368.

FIG. 4 illustrates a patient external device 400 that provides a userinterface configured to allow a human analyst to interact with theepisode data. The patient external device 400 is described as an ICDprogrammer, although the methods of the invention are operable on othertypes of devices as well, such as computers or patient informationservers used in conjunction with a remote system, for example. Theprogrammer 400 includes a programming head 410 which is placed over apatient's body near the implant site of an implanted device to establisha telemetry link between an ICD and the programmer 400. The telemetrylink allows the cardiac episode data collected by the implantable deviceto be downloaded to the programmer 400. The downloaded cardiac episodedata is stored in the programmer memory 465.

The programmer 400 includes a graphics display screen 420, e.g., LCDdisplay screen, that is capable of displaying graphics, alphanumericsymbols, and/or other information. For example, the programmer 400 maygraphically display one or more of the cardiac signals downloaded fromthe ICD on the screen 420. The display screen 420 may includetouch-sensitive capability so that the user can input information orcommands by touching the display screen 420 with a stylus 430 or theuser's finger. Alternatively, or additionally, the user may inputinformation or commands via a keyboard 440 or mouse 450.

The programmer 400 includes a data processor 460 including softwareand/or hardware for managing cardiac episode data stored in the memory465 of the programmer 400. In one implementation, cardiac episode datais received from an ICD via communications circuitry 466 of theprogrammer 400. The data processor 460 identifies cardiac episode typesbased on one or more discriminating features of the tachyarrhythmiaepisodes detectable in the cardiac episode data. The data processor maycompare features of induced tachyarrhythmia episodes to those ofidentified episode types. The tachyarrhythmia episodes associated witheach episode types may be counted and/or ranked by the data processor aspreviously described. Information related to the comparison of inducedtachyarrhythmia episodes to identified episode types, tachyarrhythmiaepisode count and/or rankings are presented to the user via a displayscreen 420.

In one embodiment, the data processor 460 may algorithmically group thestored tachyarrhythmia episodes having similar characteristics intoepisode types. The characteristics used for grouping the episode typesmay be identified by the user or may be identified by the data processor460, for example.

In some embodiments, the groupings of similar episodes may be determinedby a user and entered via the keyboard 420, the mouse 450, or stylus 430for touch sensitive display applications. Methods and systems forgrouping tachyarrhythmia episodes are described in U.S. Pat. No.6,091,990 which is incorporated herein by reference.

In one embodiment, the data processor 460 or other remotepatient-external device, processes tachyarrhythmia episode data storedin the ICD and downloaded to the remote device. The data processor 460finds a tachyarrhythmia episode having a set of discriminating features,labels that episode as corresponding to an episode type and searches foradditional episodes with discriminating features similar to the firstepisode type. If episodes having similar discriminating features arefound in memory, the data processor 460 may count the number of similarepisodes. The number of tachyarrhythmia episodes of a particular typemay be displayed along with labels for the episode types on theprogrammer display 420.

ICDs are typically capable of acquiring cardiac EGM signals frommultiple sensing vectors, including far field and near field sensingvectors. The morphological patterns present in the individual beatsignals of a first tachyarrhythmia episode may be compared to themorphological patterns of a second one or more tachyarrhythmia episodesusing information from both the far field and near field signals of thetachyarrhythmia episodes. In one implementation, the far field signal ofa representative beat of the first tachyarrhythmia episode is alignedwith the far field signal of a representative beat of a secondtachyarrhythmia episode using fiducial points identified in the nearfield signals of the first and second tachyarrhythmia episoderepresentative beats. After alignment, amplitude values of therepresentative beat signals are then compared to determine morphologicalsimilarity between the first and second tachyarrhythmia episodes.

In one example, predetermined feature points F₁-F₅ may be extracted froma representative or composite far field signal beat of the firsttachyarrhythmia episode as illustrated in FIG. 5A. After alignment usingrate channel signals, illustrated in FIG. 5B, corresponding samples ofthe representative or composite beat of a second tachyarrhythmia episodeare extracted and compared to the features of the first tachyarrhythmiaepisode. For example, the comparison may involve calculating a featurecorrelation coefficient (FCC) which quantifies the similarity betweenthe signal morphology of the first and second tachyarrhythmia episodes.The FCC may be compared to a predetermined threshold to determine ifmorphology of the tachyarrhythmia episodes is similar or dissimilar.

The diagram of FIG. 6 illustrates a process for identifying episodetypes and counting tachyarrhythmia episodes associated with episodetypes in accordance with one embodiment. Near field and far fieldcardiac signals of tachyarrhythmia episodes are acquired and stored 610in the ICD. For example, the near field signal may be acquired via aRV-tip to RV-ring sensing vector and the far field signal may beacquired via the SVC coil to can sensing vector.

The tachyarrhythmia episode signals are downloaded 620 to a deviceprogrammer or other remote patient-external computer. Features of thefar field signal of a representative beat of a first tachyarrhythmiaepisode are extracted 630. The first tachyarrhythmia episode detected islabeled and counted as a new episode type. A next tachyarrhythmiaepisode is detected. A representative beat signal of the nexttachyarrhythmia episode is aligned 640 with the representative beat ofthe first tachyarrhythmia episode using their corresponding rate channelfiducial points. The features of the far field signals of thetachyarrhythmia episodes are compared 650. If the features are similar660, the next tachyarrhythmia episode is counted 670 as corresponding tothe first episode type.

If the features of the next tachyarrhythmia episode are not similar 660to the features of the first tachyarrhythmia episode, and there are moreepisode types to compare 680 to the tachyarrhythmia episode features,then the features of the tachyarrhythmia episode are compared to one ormore additional episode types 690. If all the episode types have beencompared to the tachyarrhythmia episode and none are similar, then thetachyarrhythmia episode is labeled and counted 685 as a new episodetype. The process illustrated in the diagram of FIG. 6 continues untilall tachyarrhythmia episodes have been counted.

While the labeling and counting process described in connection withFIG. 6 can be accomplished using sensing vectors available via a singleRV lead, a more precise characterization of tachyarrhythmia episodetypes may be possible through the use of sensing vectors of multipleleads available in biventricular ICD systems. The process described inconnection with FIG. 6 above may be based on a single sensed near fieldelectrogram occurring within a single sensed far-field electrogram. Withthe addition of an LV lead, as in the case of a biventricular ICD, theavailable electrogram signals are expanded to left and right near fieldelectrogram signals occurring in conjunction with multiple far-fieldelectrograms. For example, additional sensing vectors available from abiventricular device include the vectors in the representative listprovided in Table 1:

TABLE 1 ANODE CATHODE SVC Coil RV coil (or ring), RV tip, LV ring, or LVtip Can RV coil (or ring), RV tip, LV ring, or LV tip RV coil LV coiland LV tip RV tip LV coil and LV tip

The use of a multiple lead recording system for tachyarrhythmia episodesprovides information that may be used to determine a more precisecharacterization of all tachyarrhythmias.

Analysis of the electrogram signals of a multi-lead ICD may also beperformed to indicate exit sites of the various episode types in thesame way that analysis of a 12-lead electrocardiogram can identify a PVCor VT site of origin to a region of the heart. Analysis of the vector ona far-field EGM could regionalize the site of origin of the PVC or VT toa specific location in either the left or right ventricle. In oneembodiment of the invention, EGM signals acquired from the sensingvectors of a multi-lead biventricular ICD are analyzed to determine exitsites for each of episode type. A process for determining exit sites forone or more episodes types is illustrated by the diagram of FIG. 7.Multiple near field and far field electrogram signals of tachyarrhythmiaepisodes are acquired and stored 710 by an ICD. Periodically or oncommand, the EGM data are downloaded 720 to a device programmer or otherremote, patient-external device. The episode types are identified 730 aspreviously described. For each episode type, the multiple EGM signalsare analyzed 740 to determine an exit site of the episode type. Thisprocess continues 750 until the exit site for each identified episodetype has been analyzed 760. The exit site information is displayed 770,optionally along with additional information including the episode typelabels a number of tachyarrhythmia episodes detected for each episodetype and/or rankings of the episode types.

Location of PVC or VT exit site, e.g., lateral wall, apex, or non-septalsite may be characterized by distinct patterns when compared to LVpacing, RV pacing and sinus rhythm. For example, if a particular episodetype morphology resembles LV pacing, the exit site for that episode typecan be assumed to be near the LV pacing site. The process describedabove may be modified to alternatively or additionally include analysisof signals produced by LV pacing, RV pacing, and sinus rhythm todetermine the exit site for the tachyarrhythmias.

In accordance with various embodiments, the number of episode types isalgorithmically determined. For example, the episodes may be clusteredusing a genetic algorithm, such as a K-means clustering algorithm. Inone embodiment involving K-means clustering, each episode's Ndistinguishing features are expressed as an N-dimensional featurevector. The feature vectors of all episodes can be thought of as beingplotted in N-dimensional space. Episodes similar in conduction patternwill be plotted close to each other while episodes having different inconduction pattern will be plotted farther from each other. The K-meansclustering algorithm will automatically cluster the episodes into Kgroups, with each group represented by its mean feature vector and eachepisode assigned to the group with the closest mean feature vector.

In some embodiments, groups may be formed where a particular episode canbe a member of only one group. In other embodiments, a particularepisode may be a member in more than one group. For example, a fuzzyK-means clustering algorithm may be used to group the episodes. Withconventional K-means clustering described above, an episode can belongto only one cluster as the conventional algorithm assigns to an episodeone crisp membership value per cluster (equal to 0 if the episode is nota member of the cluster and 1 if the episode is a member of thecluster). In contrast, with the fuzzy algorithm, an episode can belongto several clusters, as the fuzzy algorithm assigns to an episode onemembership value per cluster ranging from 0 to 1. These membershipvalues specify the episode's degree of membership into each cluster.

Note that in the above-described embodiments, the number of clusters isfirst determined before using the genetic K-means algorithm to clusterthe episodes. In some implementations, the physician specifies K whichis the number of groups or clusters. In other implementations, theprocessor uses a predetermined number as K. In yet otherimplementations, the processor determines the value of K by partitioningthe N-dimensional space based upon the density of the feature vectors inthe space. In a further implementation, the processor determines thevalue of K by partitioning the N-dimensional space based upon a matrixof similarity measurements comparing each feature vector to each other.

In one scenario, the processor may use a probability function such as aprobability density function (PDF) or a cumulative distribution function(CDF) to determine the number of clusters. This technique is describedwith reference to determining the number of rate zones in commonly ownedU.S. patent application Ser. No. 11/506,253, filed Aug. 18, 2006 whichis incorporated herein by reference. The same principle is applicable todetermining the number of clusters based on the PDF or CDF formed usingdiscriminating features or mean feature vectors of the episodes. Thenumber of groups may be determined from the morphology of the CDF or PDFas is described for determining the number of rate zones in thepreviously incorporated patent application.

In yet other embodiments, the episodes are first ordered and are thengrouped. For example, the episodes may first be ordered based on thevalue of one discriminating feature or may be ordered based on Ndiscriminating features. A physician may identify groups of the orderedepisodes. In one scenario, the physician may identify episodes, denotedherein as border episodes. The border episodes may have one or morediscriminating features that appear to form a natural division betweenone group and another group, for example. After determination of thenumber of episode types, counting and/or ranking the episode types mayproceed as described above.

As the amount of information collected and stored in ICDs increases, theinterpretation of data becomes more complex and time consuming. Thepresent invention provides methods and systems for arranging cardiacepisodes based on discriminating features of the episode data. Theprocesses described herein allow a physician to more easily identifycardiac episodes types and determine the VT burden associated withdifferent episode types. The methods of the present invention allow aphysician to identify the number and/or frequency of different types ofcardiac episodes experienced by the patient. Data processing algorithmsused to identify episode types, count tachyarrhythmia episodesassociated with episode types and rank episode types to determine the VTburden of the various episode types may operate in an implantable orpatient-external device that also acquires the episode data. In anotherversion, an implantable or non-implantable device may acquire theepisode data and the arrangement and/or labeling of the data may beperformed by a device programmer or a remote server, for example.

Various modifications and additions can be made to the preferredembodiments discussed hereinabove without departing from the scope ofthe present invention. Accordingly, the scope of the present inventionshould not be limited by the particular embodiments described above, butshould be defined only by the claims set forth below and equivalentsthereof.

1. A method of operating a cardiac system, comprising: sensing andstoring electrogram signals of cardiac electrical activity in animplantable cardiac device; detecting tachyarrhythmia episodes using theelectrogram signals; identifying tachyarrhythmia episode types based oncharacteristics of the electrogram signals; inducing a tachyarrhythmiaepisode in preparation for performing ablation; comparingcharacteristics of the induced tachyarrhythmia episode tocharacteristics of the identified episode types; and indicating asimilarity between the induced tachyarrhythmia episode and at least oneof the episode types identified from the stored electrogram signals tofacilitate performing the ablation.
 2. The method of claim 1, furthercomprising: counting a number of stored tachyarrhythmia episodesassociated with each episode type; and displaying a number oftachyarrhythmia episodes counted for each episode type.
 3. The method ofclaim 1, further comprising ranking the episode types based on thenumber of tachyarrhythmia episodes counted for each episode type, a rankof an episode type corresponding to a tachyarrhythmia burden of theepisode type.
 4. The method of claim 1, further comprising performingablation to prevent future occurrences of one or more of the identifiedepisode types.
 5. The method of claim 1, further comprising comparingepisode types identified from tachyarrhythmia episodes stored beforeablation to episode types identified from tachyarrhythmia episodesstored after ablation.
 6. The method of claim 1, wherein thecharacteristics of the tachyarrhythmia episodes of each episode type areassociated with a conduction pattern of the episode type.
 7. The methodof claim 1, wherein the characteristics of the tachyarrhythmia episodescomprise one or more morphological characteristics of thetachyarrhythmia episodes.
 8. The method of claim 1, wherein thecharacteristics of the tachyarrhythmia episodes comprise one or moreinterval characteristics of the tachyarrhythmia episodes.
 9. The methodof claim 1, further comprising ranking the tachyarrhythmia episodetypes, wherein the criteria used for the ranking is user programmable.10. The method of claim 1, further comprising determining a number ofdistinct episode types.
 11. The method of claim 1, wherein determiningthe number of distinct episode types comprises: identifying, by a user,one or more episodes defining boundary episodes between the episodetypes; and grouping the tachyarrhythmia episodes based on the boundaryepisodes.
 12. The method of claim 1, wherein a number of distinctepisode types is determined by a user.
 13. The method of claim 1,wherein a number of distinct episode types is determinedalgorithmically.
 14. The method of claim 1, wherein: sensing theelectrogram signals comprises sensing a far field electrogram signalusing a far field sensing vector and a near field electrogram signalusing a near field sensing vector; and identifying the tachyarrhythmiaepisode types comprises identifying the tachyarrhythmia types based onmorphological or interval characteristics determined using the far fieldsignal and the near field signal.
 15. The method of claim 1, wherein:sensing the electrogram signals comprises sensing a plurality of nearfield electrogram signals and a plurality of far field electrogramsignals; and identifying the tachyarrhythmia episode types comprisesidentifying the tachyarrhythmia types based on morphological or intervalcharacteristics determined using at least one of the plurality of farfield signals and at least one of the plurality of near field signals.16. The method of claim 1, further comprising determining exit sites ofthe identified episode types using the electrogram signals.
 17. Acardiac system, comprising: an implantable sensing system configured tosense and store electrogram signals; a tachyarrhythmia detectorconfigured to detect tachyarrhythmia episodes using the electrogramsignals and to determine characteristics of the tachyarrhythmia episodesfrom the electrogram signals; a data processor configured to identityepisode types based on the characteristics of the tachyarrhythmiaepisodes, the data processor further configured to determine asimilarity between a tachyarrhythmia episode induced in preparation forperforming ablation and at least one identified episode type; and adisplay configured to present information related to the similaritybetween the induced tachyarrhythmia episode and the at least oneidentified episode type to facilitate performing the ablation.
 18. Thesystem of claim 17, wherein the data processor is configured to rank theepisode types, a rank of an episode type corresponding to atachyarrhythmia burden of the episode type.
 19. The system of claim 17,wherein the data processor is configured to rank each episode type basedon a number of tachyarrhythmia episodes counted for the episode type.20. The system of claim 17, wherein the characteristics of thetachyarrhythmia comprise morphological characteristics.
 21. The systemof claim 17, wherein the characteristics of the tachyarrhythmia episodesof each episode type characterize timing interval characteristics. 22.The system of claim 17, wherein the data processor is further configuredto determine a number of distinct episode types.
 23. The system of claim17, wherein the data processor is configured to discriminate a firsttype of ventricular tachyarrhythmia episode from a second type ofventricular tachyarrhythmia episode.
 24. The system of claim 17,wherein: the sensing system comprises: a first pair of cardiacelectrodes configured to sense a far field electrogram signal; and asecond pair of cardiac electrodes configured to sense a near fieldelectrogram signal; and the tachyarrhythmia detector is configured todetermine the characteristics of the tachyarrhythmia episodes using thefar field electrogram signal and the near field electrogram signal. 25.The system of claim 17, wherein: the sensing system comprises: aplurality of far field cardiac electrode pairs, each far field electrodepair configured to sense a far field electrogram signal; and a pluralityof near field cardiac electrode pairs, each near field electrode pairconfigured to sense a near field electrogram signal; and thetachyarrhythmia detector is configured to determine the characteristicsof the tachyarrhythmia episodes using at least one of the plurality offar field electrogram signals and at least one of the plurality of nearfield electrogram signals.
 26. The system of claim 17, wherein the dataprocessor is further configured to determine an exit site of one or moreof the identified episode types based on the electrogram signals. 27.The system of claim 17, wherein the data processor is further configuredto compare characteristics of the tachyarrhythmia episodes toelectrogram signals of paced beats to determine an exit site of one ormore identified episode types.
 28. A system, comprising: an implantablesensing system configured to sense electrogram signals; atachyarrhythmia detector configured to detect tachyarrhythmia episodesusing the electrogram signals and to determine characteristics of thetachyarrhythmia episodes from the electrogram signals; means foridentifying episode types based on the characteristics of thetachyarrhythmia episodes; and means for determining a similarity betweenan induced tachyarrhythmia episode and one or more identified episodetypes.
 29. The system of claim 28, further comprising means fordetermining a number of distinct episode types.
 30. The system of claim28, further comprising means for determining an exit site for eachepisode type based on the electrogram signals.